Method of finishing piezoelectric crystals



Oct. 16, 1945. H. F. FRUTH 2,387,137.

7 METHOD OF FINISHING PIEZOELECTRIC CRYSTALS Filed June 28, 194':

35 0,0 SOURCE INVENTOR HAL F. FR UTH RNEY METHOD OF FINISHING PIEZOELECTRIC CRYSTALS Hal F. Fruth, Chicago, Ill., assignor to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application June 28, 1943, Serial No. 492,540

6 Claims.

The present invention relates to improved methods for grinding piezoelectric crystals and more particularly to improved methods and ma terials for grinding piezoelectric crystals and for removing oil, grease or like substances from the surfaces of piezoelectric crystals both before and afterthe grinding operations are completed. This application is a continuation-in-part of copending application Serial No. 479,928, filed March 20, 1943.

In the manufacture of piezoelectric crystal from quartz crystal stock, for example, the crystal blanks are first diced or cut from the mother crystal after which they are rough ground to the approximate desired dimensions. Thereafter,

the faces and edges of the crystal are finish ground substantially to the exact desired dimensions by using a grinding or abrading material of suitable fineness. As will appear more fully hereinafter, it is frequently desirable, particularly in the cutting and rough grinding operations, to utilize oil, grease or a similar adhesive substance, for the purpose of facilitating the cutting and grinding operations. The oil or grease used for this purpose is deposited on the surfaces of the crystal during the operations referred to and must be removed from the finished crystals. Moreover, any oil films or deposits remaining on the crystal surfaces after the cuttin and rough grinding operations are completed should also be removed before the finish grinding operations are started, since otherwise the grinding material used for finish grinding may be contaminated with particles of the previously used rough grinding material to produce scratching of the crystal surfaces during the finish grinding operations. Due to the high adhesion of oil and like substances, and the degree with which such substances penetrate the crystal surface pores, it is exceedingly difficult to thoroughly remove the oil films or deposits and the minute particles suspended therein from the crystal surwithout etching or otherwise damaging the cr'ys-' tal surfaces.

According to another object of the invention, the surfaces of the crystals arethoroughly and automatically scrubbed during the cleaning operation in order to remove therefrom any loose crystals which is fast, thorough and reliable, is"

free of all me and health hazards, and is in no way disagreeable to persons who may practice the same.

According to another object of the invention,

an improved method of cleaning piezoelectric crystals is provided which has the effect of changing the frequency characteristics of crystals cleaned thereby.

. According to still'another object of the invention, the finish grinding of the crystals is enhanced and contamination of the abrading material used for finish grinding is prevented by providing an improved and exceedingly simple method of cleaning the crystals after the cutting and rough grinding operations are .completed and before the finish grinding of the crystals is started. g

It is a further object of the invention to provide a cheap and fast acting material for removing films of oil or like substances from the surfaces of piezoelectric crystals.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawing, in which the single feature is a sidesectional view of improved apparatus. for tumbling a mixture of crystal blanks in the presence of an abrasive or cleaning material in order member II, a rotatable shaft 15., and a speed.

reducing gear train connecting the drive shaft of the motor and the shaft I5. The shaft I functions to support and rotate a tumbler cup I6 which is located exteriorly of the carriage II. This-cup is preferably of square cross-section along the body thereof and is provided with a round neck IBa of reduced diameter which is suitably threaded at the end to receive an internally threaded cover 22. More specifically, the shaft I5 is journaled in bearing plates I'Ia and I'Ib suitably secured to the rear and front walls Na and Ill), respectively, of the carriage II. Suitable collars I8aand lb, which may be slipped over the shaft and set in a given position by means of set screws or the like, are provided adjacent the bearing plates Ila and I'Ib, respectively, in order to limit axial movement of the shaft I5. A cup supporting plate I9 is rigidly mounted upon the right end of the shaft I5 and carries four outwardly projecting, angularly spaced-apart resilient arms 20, which in combination, define a nest for receiving the cup IS. The four arms 20 may be bent over from an annular base 2 I, so that a one-piece construction is provided, and the annular base 2| may be suitably secured to the outer surface of the plate I9 by spot welding the inner edge thereof to the adjacent plate surface. With the tumbler cup I6 nested within the resilient arms 20, the bent over end portions 200. of the arms 20 engage the base of the neck I6a to hold the cup IS in place. If desired, a rubber band 2Ia may be used to increase the pressure of the arms 20 against the side walls of the cup I6.

For the purpose of imparting rotary movement to the shaft I5, thereby to rotate the tumbler cup IS, a motor 24 supported upon the member I4 is provided. This motor is arranged to drive the shaft I5 through a speed reducing gear train which comprises the gears 25, 26, 21 and 28. The two intermediate gears 26 and 21 of this train are suitably mounted for rotation with a shaft 29 which is journaled at its ends in the side walls of the member I4. It will be understood in this regard, that the gear 25 which meshes with the driven gear 26 is rigidly mounted upon the end of the motor shaft and that the driven gear 28 is rigidly mounted upon the tumbler drive shaft I5. Set screws or bolts 30 extending through holes drilled in the side walls Ila and H17 of the carriage II and threaded into the side walls of the member I4 may be utilized for supporting the motor 24 and the speed reducing gear train within the carriage I I.

Although normally the apparatus is operated with the axis of rotation of the cup I6 disposed in a horizontal plane, it may be desirable to adjustably tilt the carriage II with reference to a horizontalv surface upon which the base I0 is supported, thereby to change the angle of inclination of the tumbler cup IS with reference to the horizontal. To this end adjusting means comprising a link 3| and a wing nut assembly 32 may be provided. More specifically, the lower end of the link 3| is pivotally connected to an edge surface of the base III by means of a pivot screw 33, and is provided with a slot 34 in which a bolt 32a of the wing nut assembly 32 is slidably disposed. The base of the bolt 32a is pivotally anchored upon a lug 320 which is suitably secured to the outer face of the carriage well IIb. With this construction, the wing nut 32b may be loosened in order that the carriage II and the parts supported thereby may be tilted to any desired angular position, and then tightened to v for each crystal.

retain the carriage II and the tumbler cup I6 in the selected position.

Although the motor 24 may be of any desired adjustable speed type, it is preferably an adjustable speed direct current motor, and may be connected for energization from a direct current source connected to the terminals 35. An adjustable rheostat 36 is provided in one side of the circuit for energizing the motor 34 in order that the speed of operation of the motor may be varied as desired.

In utilizing the above-described apparatus to practice the present improved method, piezoelectric quartz crystal blanks which have been cut from the crystal stock and then machine ground to rectangular dimensions only slightly larger than those desired, are lap ground to a thickness approximately .0001 inch greater than the desired thickness and are then cleaned in accordance with the improved cleaning method more fully described below for a period of ap: proximately one-half hour. The cleaned crystals are next mixed with a charge of abrasive material in the tumbler cup I6, and the cup; with the cover 22 fitted over the open end thereof, is inserted between the arms 20. Operation of the motor 24 to rotate the tumbler cup at the speed established by the setting of the rheostat 36a is now initiated. During rotation of the tumbler cup, random relative movement occurs between the crystals and the abrasive material so that all surfaces of the crystals are ground or abraded. The extent of the grinding action per unit time interval is of course determined by the speed of the tumbling action as established by the speed of rotation of the cup I6, and by the character and fineness of the abrasive material used.

More specifically considered, the finish grinding of a given batch of quartz crystals is usually carried out in a number of steps. During the first or primary step, the crystals are ground at a relatively high grinding speed to approach the particular resonant frequency characteristic desired In this regard it will be understood that the crystal blanks as initially cut and machine ground are of such dimensions that the resonant frequency of each crystal is substantially below the particular resonant frequency which is desired. During the primary grinding step, therefore, the extent of grinding is limited not to exceed an amount which will bring any of the crystals to a resonant frequency higher than that desired. It has been found that the primary grinding step may be satisfactorily carried out by mixing the blanks with a charge of abrasive material consisting of No. 320 screen mesh silicon carbide. In order to provide the required contact pressure between the surfaces of the crystals and the abrasive material which is necessary to obtain the desired relativel fast cutting during the primary grinding operation, a suitable body material is added to the mix. This body material may consist of a suitable amount of gravel, the particles of which have diameters ranging from .125 inch to .25 inch. For the purpose of causing the relatively fine abrasive particles to adhere to the surfaces of the relatively coarse grains of gravel, a suitable liquid adherent, such, for example, as light oil or the like, is preferably added to the mix. In the specific example given above, approximately 20 c. c. of light oil is added to the mixture. This liquid adherent not only causes the relatively fine abrasive particles to adhere to the surfaces of the coarse grains of gravel, but incidentally forms an adhering film on the exposed surfaces of the crystals which are being ground.

As indicated above, after the mixing operation is completed, the desired primary grinding of the crystal'blanks is obtained by utilizing apparatus of the character illustrated in the drawing to turnblethe mixture for a predetermined time interval at" a preestablished tumbling rate. In this grinding operation, the relative movement between the fine abrasive material and the coarse body material, i. e. the gravel, soon causes the surfaces of the body material'to be coated with the abrasive material, the oil acting to enhance the coating action and to maintain the abrasive coating on each grain of body material. As the tumbling of the mixture proceeds, the crystals are wholly free tomove at random through the coated grains of gravel, contact at all times being maintained between two or more surfaces of each crystal and a relatively large number of abrasive coated grains of gravel. When the apparatus illustrated in the drawing is utilized to tumble the mixture, stratification of the crystals in certain zones of the mixture is completely avoided, with the result that the extent of the random movements of the many crystals is about the same. Accordingly, the crystals are ground at substantially the same rate. In fact, using the particular mixture described above, it has been found that the resonant frequencies of the crystals change at the substantially uniform rate of approximately 700 cycles per hour when the tumbling cup of the apparatus in use is rotated about a horizontal axis at a speed of twenty-six revolutions per minute. If, therefore, the approximatev resonant frequencies of the crystals are known before finish grinding is started and the particular resonant frequency to which each crystal is to be ground is also known, the time interval during which the mixture is tumbled during the primary grinding step may be so chosen that the resonant frequencies of all crystals in the batch are raised to within a given number of cycles of the desired frequency.

After the primary grinding operation is completed in the manner explained above, the crystals are measured to determine the frequency characteristics thereof, and are grouped or classified according to their measured frequencies. In other words, those crystals having substantially equal resonant frequencies, as determined by conventional methods of frequency measurements, are placed in the same group or class. The groups, according to frequency, may be approximately 500 cycles apart. In the usual case, certain of the crystals in the batch will be found to have been ground to resonant frequencies which are so close to the desired frequency as to require no secondary grinding. Thus crystals having frequencies approximately 500 cycles below the desired frequency, will ordinarily require no further grinding. The remaining crystals, 1. e., those having resonant frequencies well below the desired value, as grouped in the manner explained above, are subjected to further grinding at a considerably lower grinding speed. To this end, the crystals of the respective groups are separately mixed with different charges of abrasive material. It has been found that No. 220 screen mesh silicon carbide is entirely satisfactory for this purpose. The separate mixes are made in the different tumbling cups of difi'erent tumbling mills of the character shown in the drawing, and each mix is separately tumbled or agitated at a predetermined tumbling rate for a particular time interval which is dependent upon the difference between the average resonant frequency of the particular class or group of crystals and the desired resonant frequency. Assuming, for example, that there are three different crystal groups which respectively contain crystals of different resonant frequencies, and also that the same tumbling speed is used for the different mixes, three different intervals of tumbling time are required for the three different mixes.

After each mix is tumbled for a time interval, such that the frequencies of the crystals therein are raised to a value approximately 500 cycles below the desired value, the crystals are separated from the abrasive material by screening or otherwise, after which they are cleaned by utilizing the novel cleaning method described below. After they have been cleaned, the crystals may be individually tested to determine the exact frequency characteristics thereof.

As indicated above, both before and after the finish grinding operations are completed, the crystals are subjected to the present improved cleaning method in order to remove any films of the abrasive adherent and particles of crystal or grinding material which may remain upon the surfaces thereof. More specifically, and as will be apparent from the preceding explanation, the surfaces of the crystals may, during the cutting, rough grinding, lapping and finish grinding operations, become coated with the oil or other adhesive substance which may be used to enhance the cutting and grinding operations. Thus the oil which may be added to the first grinding mix for the purpose of causing the grains of gravel to be coated with the relatively fine abrasive material, may coat the surfaces of the crystals and cause crystal dust and particles of grinding material to adhere to the crystal surfaces. Also, the oil or adhesive substance tends to penetrate the pores of the crystal surfaces and may cause crystal dust and particles of the abrasive material to adhere to the crystal surfaces after the crystals are segregated from the abrasive mixture. The film or deposit of the particular adhering substance which is used, as well as the crystal dust and particles of abrasive material which may adhere to the crystal surfaces, must be removed before the crystals are placed in operation. It is also desirable to remove the adherent films or deposits before the finish grinding operations are started, since otherwise, the particles of abrasive material used in the rough grinding and lapping operations may contaminate the material which is used in the finish grinding operations.

For the purpose of cleaning the crystal surfaces, a cleaning material is used which comprises a grit-like material intermixed with a suitable wetting agent solution. the crystals being introduced into the mixture and then agitated in a tumbling mill of the character illustrated in the drawing. More specifically, the cleaning material may consist of a one-tenth of one per cent solution of a suitable liquid wetting agent taken from the class of wetting agents having the characteristics of reducing the surface tension of oil, grease or the like substances in order to permit emulsification thereof, which is intermixed with No. 9-14 screen mesh grit-like material. The grit-like material may comprise green color quartz crystal grit or gravel which is readily obtainable in commercial quantities at low cost. It has been found that a wetting agent selling under the trade name of Aerosol is well adapted for use as the emulsifying or wetting agent. Tergitol is another wetting agent which may be used.

As to the relative quantities, it has been found that one pound of green quartz crystal grit when added to one quart of the 1 s of one percent Aerosol wetting agent solution forms a highly satisfactory cleaning material. These mixed ingredients in the defined proportions may be placed in a square two-quart tumbling jar of the character shown in the drawing, after which the batch of crystals may be added to the mixture. After the tumbling jar is aflixed to the supporting arms of the tumbling mill, the jar is rotated at a speed of thirty-five revolutions per minute for a period of one-half hour or more. During the tumbling operation, the points of the quartz grit are brought into random contact with the surfaces of the quartz crystals and slide over these surfaces with the result that the oil or adhesive films formed upon the crystal surfaces are broken up into small sections or segments. The scrubbing action of the grit also removes any loose particles from the crystals surfaces and mildly abrades the surfaces to remove any small sharp points remaining thereon. The wetting agent, namely, the Aerosol, included in the mixturefhas the effect of decreasing the surface tension of the fihns formed upon the crystal surfaces, permitting the oil or other adhesive substance'of which these films are formed to be emulsified in the solution. The breaking up of the films into small segments or sectors of course materially enhances the action of the wetting agent in emulsifying the adhesive substance from which the films are formed and thus permit the emulsifying action to proceed more rapidly. Also, the random contacting of the crystal surfaces with the grit and the slight liquid pressure pulsations produced at the crystal pore openings, causes any abrasive particles lodged in the pores of the crystal surfaces to become dislodged so that they may be removed by washing.

After the crystals have been agitated in the cleaning material for a suitable measured time interval, they are separated from the cleaning material by pouring the mixture on the screen' of suitable mesh. The separated crystals are rinsed in tap'water and finally in distilled water, following which they are oven dried at a suitable drying temperature while still on the screen.

It has been found in practice that the abovedesribed cleaning action is accompanied by slight but definite changes in the resonant frequencies of the crystals. More particularly, the resonant frequencies of crystals which have been finish ground are increased approximately 500 cycles when subjected to the cleaning process for a period of approximately one-half hour. The predominant portion of the frequency change occurs during the first ten minutes of the halfhour period. During the remaining portion of the period. the frequency change which occurs is relativel small and is substantially constant per unit time interval. Thus the process is well adapted for use as the final step in raising the resonant frequencies of the crystals to the exact desired values.

From the foregoing explanation it will be understood that by utilizing the present improved method of grinding and cleaning piezoelectric crystals, the cost of producing these crystals on a production basis is reduced to an exceedingly low figure. It will be understood, moreover, that the novel cleaning method herein disclosed is strictly a low cost process requiring relatively cheap material and substantially no labor in the practice thereof. The cleaning action produced by practicing this method is completely thorough, all traces of oil, grease or other foreign material being completely removed from the crystal surfaces. In this regard it will be understood that the removal of extraneous material which may adhere to the crystal surfaces after the cutting and rough grinding operations are completed, materially enhances the accuracy with which the crystal frequencies may be measured and, accordingly, the accuracy with which the calculated time intervals for the finish grinding operations may be computed. Thus the novel cleaning method herein disclosed contributes materially to the speed with which the crystals may be produced and to the accuracy with which they may be ground to specified and desired frequencies.

While one embodiment of the invention has been disclosed, it will be understood that various modifications may be made therein, which are within the true spirit and scope of the invention.

I claim:

1. The process of producing crystals of a desired frequency from unfinished crystals of lower frequency and of concurrently cleaning the crystals, which comprises the steps of mixing the crystals with a liquid cleaning agent and a charge of loose grit-like material capable of abrading the crystal faces, and agitating the mixture to produce relative sliding movement between the crystals and the grit-like material, thereby to effect an increase in the resonant frequencies of the crystals toward said desired frequency and concurrent washing of the crystal surfaces.

2. The process of producing c ystals of a desired frequency from unfinished crystals of lower frequency and of concurrentl cleaning the crystals, which comprises the steps of mixing the crystals with a charge of loose grit-like material capable of gently abrading the crystal faces and a liquid cleaning solution consisting at least in part of an oil wetting agent, and tumbling the mixture to produce agitation of the solution and crystals and relative sliding movement between the crystals and the grit-like material, thereby to effect an increase in the resonant frequencies of the crystals toward said desired frequency and concurrent cleansing of the crystal surfaces to remove oil deposits and loose solid particles therefrom.

3. The method of grinding piezoelectric crystals to establish a predetermined frequency characteristic for each crystal, which comprises mixing a batch of crystals with a charge of abrasive material, agitating the mixture to produce relative movement between the crystals and the abrasive material, separating the crystals into groups according to the frequency characteristics thereof, separately mixing thecrystals of certain of said groups with difierent charges of abrasive materiah and separately agitating the different mixes to produce relative movement between the crystals of each group and the abrasive material of the mixture.

4. The method of grinding piezoelectric crystals to establish a predetermined frequency characteristic for each crystal, which comprises mixing a batch of crystals with a relativel fine abrasive material and a coarse body material, agitating the mixture to coat the body material with the abrasive material and to produce random relative movement between the coated body material and the crystals, separating the crystals into groups according to the frequenc character-- istics thereof, after the mixture has been agitated for a selected time interval, separately mixing the crystals of certain of said groups with different charges of abrasive material, and sepa rately agitating-said last-named mixtures to produce random relative movement between the crystals oft-each group and the abrasive material.

5. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals of lower frequency, which comprises the steps of mixing a batch of the unfinished crystals with a mass of loose granular abrading material, agitating the mixture to flow the mixture and thus cause the crystals to migrate about in and slide relative to the abrading material with low contact pressure engagement therebetween, separating the crystals from the abrading material, measuring the resonant frequencies of the crystals, classifying the crystals in groups according to the measured resonant frequencies thereof, separately mixing the crystals of at least certain of said groups with different charges of loose granular abrading material, and separately agitating the different mixes to produce the same crystal abrading action as that produced during the initial abrading step, thereby further to increase the resonant frequencies of the crystals toward said desired frequency.

6. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals of lower frequency, which comprises the steps of mixing a batch of the unfinished crystals with a mass of loose granular abrading material, agitating the mixture to flow the mixture and thus cause the crystals to migrate about in and slide relative to the abrading material with low contact pressure engagement therebetween, separating the crystals from the abrading material, measuring the resonant frequencies of the crystals, classifying the crystals in groups according to the measured resonant frequencies thereof, separatel mixing the crystals of at least certain of said groups with different charges of loose granular abrasive material, and separately agitating for different time intervals the different mixes to produce the same crystal abrading action as that produced during the initial abrading step, thereby further to increase the resonant frequencies of the crystals toward said desired frequency, said different time intervals being directly related to the differences between said desired frequency and the average resonant frequencies of the crystals of the different groups.

HAL F. FRUTH. 

